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| United States Patent |
5,712,592
|
|
Stimson
, et al.
|
January 27, 1998
|
RF plasma power supply combining technique for increased stability
Abstract
An RF power amplifier including a splitter, two branch circuits, and a
combiner. The splitter has an input line receiving an RF signal, a first
output line carrying a first output signal derived from the RF signal, and
a second output line carrying a second output signal derived from the RF
signal. A first branch circuit receives the first output signal and
generates therefrom a first derived signal. A second branch circuit
receives the second output signal and generates therefrom a second derived
signal therefrom. The first branch circuit includes a first power
amplifier and a phase shifting element. The second branch circuit includes
a second power amplifier. The combiner, which has a first input receiving
the first derived signal and a second input receiving the second derived
signal, combines the first and second derived signals to produce a power
output signal therefrom. The phase shifting element is connected between
the output line of the first power amplifier and the first input of the
combiner and produces a phase shift in a signal passing from the first
power amplifier to the combiner relative to a signal passing from the
second power amplifier to the combiner.
| Inventors:
|
Stimson; Bradley O. (Mountain View, CA);
Rummel; Paul W. (Lynn, MA)
|
| Assignee:
|
Applied Materials, Inc. (Santa Clara, CA)
|
| Appl. No.:
|
398426 |
| Filed:
|
March 6, 1995 |
| Current U.S. Class: |
330/124R; 315/111.51; 330/295 |
| Intern'l Class: |
H03F 003/68 |
| Field of Search: |
330/124 R,295
315/111.51
|
References Cited
U.S. Patent Documents
| 4629940 | Dec., 1986 | Gagne et al. | 315/111.
|
| 4701716 | Oct., 1987 | Poole | 330/124.
|
| 4965527 | Oct., 1990 | Clark et al. | 330/124.
|
| 5101171 | Mar., 1992 | Redmond | 330/124.
|
| 5394061 | Feb., 1995 | Fujii.
| |
| Foreign Patent Documents |
| A-2 310 656 | Dec., 1976 | FR.
| |
| B11 60 904 | Feb., 1962 | DE.
| |
| 194611 | Nov., 1982 | JP | 330/124.
|
| 4057406 | Jun., 1990 | JP.
| |
| 4104603 | Apr., 1992 | JP | 330/295.
|
| 1497718 | Jul., 1989 | SU | 330/295.
|
Primary Examiner: Mullins; James B.
Attorney, Agent or Firm: Fish & Richardson P.C.
Claims
What is claimed is:
1. An RF power supply for delivering RF power to a plasma, said supply
comprising:
an RF signal generator generating an RF signal having a frequency of
r.sub.RF ;
a splitter having an input line receiving said RF signal, a first output
line carrying a first output signal derived from said RF signal, and a
second output line carrying a second output signal derived from said RF
signal;
a first branch circuit receiving said first output signal and generating a
first derived signal therefrom, said first branch circuit comprising a
first power amplifier and a phase shifting element, said first power
amplifier having an output line;
a second branch circuit receiving said second output signal and generating
a second derived signal therefrom, said second branch circuit comprising a
second power amplifier having an output line;
a combiner having a first input and a second input, said first input
receiving said first derived signal and said second input receiving said
second derived signal, said combiner combining said first and second
derived signals to produce a power output signal therefrom,
wherein said phase shifting element is connected between the output line of
the first power amplifier and the first input of the combiner and produces
a first phase shift in a signal passing from the first power amplifier to
the combiner relative to a signal passing from the second power amplifier
to the combiner, and wherein said first and second branch circuits also
produce at the frequency r.sub.RF a total phase shift in the first derived
signal relative to the second derived signal that is equal to one of the
following two values: s(360.degree.)+360.degree. and
s(360.degree.)+180.degree., wherein s is an integer which may take on any
positive value or zero.
2. The RF power supply of claim 1 wherein the first phase shift is
sufficient to prevent an oscillatory instability in the RF power amplifier
when supplying power to a plasma.
3. The RF power supply of claim 1 wherein the first phase shift is about
45.degree.+n(180.degree.), where n is an integer.
4. The RF power supply of claim 1 wherein the first phase shift is about
90.degree.+n(180.degree.), where n is an integer.
5. The RF power supply of claim 1 wherein said first branch circuit further
comprises a second phase shifting element connected between the first
output of the splitter and the first power amplifier, said second phase
shifting element producing a second phase shift which when added to the
phase shift of the first mentioned phase shifting element causes the first
derived signal to have a predefined phase relationship with respect to the
second derived signal, said predefined phase relationship being determined
by requirements of the combiner.
6. The RF power supply of claim 5 wherein the first phase shift and the
second phase shift cause the first derived signal to have the following
phase relationship with respect to the second derived signal:
s(360.degree.)+360.degree..
7. The RF power supply of claim 6 wherein the first phase shift is about
90.degree.+n(180.degree.)+m(360.degree.), where n and m are integers, and
wherein the second phase shift is about
270.degree.+n(180.degree.)+k(360.degree.), where k is an integer.
8. The RF power supply of claim 5 wherein the first phase shift and the
second phase shift cause the first derived signal to have the following
phase relationship with respect to the second derived signal:
180.degree.+s(360.degree.).
9. The RF power supply of claim 8 wherein the first mentioned phase shift
is about 90.degree.+n(180.degree.)+m(360.degree.), where n and m are
integers and wherein the second phase shift is about
90.degree.+n(180.degree.)+k(360.degree.), where k is an integer.
10. The RF power supply of claim 5 wherein the first mentioned phase
shifting element is a segment of cable of a predetermined length.
11. The RF power supply of claim 10 wherein the second phase shifting
element is a segment of cable of a predetermined length.
12. The RF power supply of claim 5 wherein the combiner is a 0.degree.
combiner.
13. The RF power supply of claim 5 wherein the combiner is a 180.degree.
combiner.
14. The RF power supply of claim 1 wherein said second branch circuit
further comprises a second phase shifting element connected between the
second output of the splitter and the second power amplifier, said second
phase shifting element producing a second phase shift which causes the
second derived signal to have a predefined phase relationship with respect
to the first derived signal, said predefined phase relationship being
determined by requirements of the combiner.
15. An RF plasma system comprising:
an RF signal generator which during operation generates an RF signal;
a splitter having an input line receiving the RF derived from said RF
signal, and a second output line carrying a second output signal derived
from said RF signal;
a first branch circuit receiving said first output signal and generating a
first derived signal therefrom, said first branch circuit comprising a
first power amplifier;
a second branch circuit receiving said second output signal and generating
a second derived signal therefrom, said second branch circuit comprising a
second power amplifier having an output line;
a combiner having a first input, a second input, and an output, said first
input receiving said first derived signal and said second input receiving
said second derived signal, said combiner combining said first and second
derived signals to produce a power output signal therefrom; and
a plasma processing chamber in which a plasma is formed during operation,
said plasma receiving the power output signal from the combiner,
wherein with a plasma present in the chamber, the first power amplifier is
characterized by a first forward power curve surface and the second power
amplifier is characterized by a second forward power curve surface and
wherein the first and second power curve surfaces are oriented with
respect each other such that changes in the load that produce an increase
in forward power level for the first power amplifier produce a decrease in
forward power level for the second power amplifier and vice versa.
16. A plasma processing apparatus comprising:
a plasma processing chamber; and
an RF power amplifier supplying power to the plasma processing chamber;
an RF signal generator connected to said RF power amplifier and supplying
an RF signal having a frequency r.sub.RF ;
said RF power amplifier comprising:
a splitter having an input line receiving said RF signal, a first output
line carrying a first output signal derived from said RF signal, and a
second output line carrying a second output signal derived from said RF
signal;
a first branch circuit receiving said first output signal and generating a
first derived signal therefrom, said first branch circuit comprising a
first power amplifier and a phase shifting element, said first power
amplifier having an output line;
a second branch circuit receiving said second output signal and generating
a second derived signal therefrom, said second branch circuit comprising a
second power amplifier having an output line;
a combiner having a first input and a second input, said first input
receiving said first derived signal and said second input receiving said
second derived signal, said combiner combining said first and second
derived signals to produce a power output signal therefrom,
wherein said phase shifting element is connected between the output line of
the first power amplifier and the first input of the combiner and produces
a first phase shift in a signal passing from the first power amplifier to
the combiner relative to a signal passing from the second power amplifier
to the combiner, and wherein said first and second branch circuits also
produce at frequency r.sub.RF a total phase shift in the first derived
signal relative to the second derived signal that is equal to one of the
following two values: s(360.degree.)+360.degree. and
s(360.degree.)+180.degree., wherein s is an integer which may take on any
positive value or zero.
17. The plasma processing apparatus of claim 16 wherein the first phase
shift is about 45.degree.+n(180.degree.), where n is an integer.
18. The plasma processing apparatus of claim 16 wherein the first phase
shift is about 90.degree.+n(180.degree.), where n is an integer.
19. The plasma processing apparatus of claim 16 wherein said first branch
circuit further comprises a second phase shifting element connected
between the first output of the splitter and the first power amplifier,
said second phase shifting element producing a second phase shift which
when added to the phase shift of the first mentioned phase shifting
element causes the first derived signal to have a predefined phase
relationship with respect to the second derived signal, said predefined
phase relationship being determined by requirements of the combiner.
20. The plasma processing apparatus of claim 19 wherein the first phase
shift and the second phase shift cause the first derived signal to have
the following phase relationship with respect to the second derived
signal: s(360.degree.)+360.degree..
21. The plasma processing apparatus of claim 20 wherein the first phase
shift is about 90.degree.+n(180.degree.)+m(360.degree.), where n and m are
integers, and wherein the second phase shift is about
270.degree.+n(180.degree.)+k(360.degree.), where k is an integer.
22. The plasma processing apparatus of claim 19 wherein the first phase
shift and the second phase shift cause the first derived signal to have
the following phase relationship with respect to the second derived
signal: 180.degree.+s(360.degree.).
23. The plasma processing apparatus of claim 22 wherein the first phase
shift is about 90.degree.+n(180.degree.)+m(360.degree.), where n and m are
integers and wherein the second phase shift is about
90.degree.+n(180.degree.)+k(360.degree.), where k is an integer.
24. The plasma processing apparatus of claim 19 wherein the first mentioned
phase shifting element is a segment of cable of a predetermined length.
25. The plasma processing apparatus of claim 24 wherein the second phase
shifting element is a segment of cable of a predetermined length.
26. The plasma processing apparatus of claim 19 wherein the combiner is a
0.degree. combiner.
27. The plasma processing apparatus of claim 19 wherein the combiner is a
180.degree. combiner.
28. A method of supplying RF power to a plasma in a plasma processing
chamber, said method comprising:
splitting an RF signal into a first output signal and a second output
signal;
power amplifying the first output signal to produce a first amplified
signal;
power amplifying the second output signal to produce a second amplified
signal;
phase shifting the second amplified signal relative to the first amplified
signal to produce a phase shifted signal;
combining the first amplified signal and the phase shifted signal to
produce a power output signal therefrom; and
delivering the power output signal to the plasma in the plasma processing
chamber.
29. The method of claim 28 wherein the phase shifting step produces a phase
shift of greater than about 45.degree..
30. The method of claim 28 wherein the phase shifting step produces a phase
shift of about 90.degree..
31. The method of claim 30 wherein the phase shifting step produces a phase
shift of about 90.degree.+n(360.degree.), where n is an integer.
32. The method of claim 28 further comprising phase shifting the second
output signal relative to the first output signal prior to amplifying said
second output signal.
33. The method of claim 32 wherein the steps of phase shifting the second
output signal and phase shifting the second amplified signal cause the
phase shifted signal to have a predetermined phase relationship with the
first amplified signal, said predetermined phase relationship being
determined by requirements of the combiner.
34. The method of claim 33 wherein the steps of phase shifting the second
output signal and phase shifting the second amplified signal cause the
phase shifted signal to have the following phase relationship with respect
to the first amplified signal: s(360.degree.)+360.degree., wherein s is an
integer which may take on any positive value or zero.
35. The method of claim 34 wherein the step of phase shifting the second
amplified signal introduces a phase shift of about
90.degree.+n(180.degree.)+m(360.degree.), where n and m are integers, and
wherein the step of phase shifting the second output signal introduces a
phase shift of about 270.degree.+n(180.degree.)+k(360.degree.), where k is
an integer.
36. The method of claim 33 wherein the steps of phase shifting the second
output signal and phase shifting the second amplified signal cause the
phase shifted signal to have the following phase relationship with respect
to the first amplified signal: 180.degree.+s(360.degree.), wherein s is an
integer which may take on any positive value or zero.
37. The method of claim 36 wherein the step of phase shifting the second
amplified signal introduces a phase shift of about
90.degree.+n(180.degree.)+m(360.degree.), where n and m are integers, and
wherein the step of phase shifting the second output signal introduces a
phase shift of about 90.degree.+n(180.degree.)+k(360.degree.), where k is
an integer.
38. The method of claim 32 wherein the step of phase shifting the second
amplified signal comprises using a segment of cable having a predetermined
length to perform said phase shifting.
39. The method of claim 38 wherein the step of phase shifting the second
output signal using a second segment of cable having a second
predetermined length to perform said phase shifting.
Description
BACKGROUND OF THE INVENTION
The application relates to RF power supplies such as are used with plasma
processing chambers.
RF excited plasma chambers can exhibit an oscillation phenomenon between
the RF power supply and plasma. In RF excited plasma systems, the
impedance of the plasma can change rapidly. As the power supplied to the
plasma changes, the impedance changes. Similarly, the RF power supply's
output power will change when it experiences a change in load impedance.
These changes can cooperate to cause a fast "run-away" scenario or
oscillation.
The oscillation, which can be seen as amplitude modulation of the RF
signal, can occur at almost any frequency that is lower than the operating
frequency depending upon the manner in which the RF power supply responds
to a rapid impedance (and thus rapid output power) excursion. If the
`run-away` condition is limited and impeded by a forward power or
reflected protection control loop, the oscillation will appear at that
loop response frequency. If the `run-away` condition simply causes a
temporary depletion of stored energy of the power amplifier of the RF
power supply, the oscillation can occur at much higher frequencies.
SUMMARY OF THE INVENTION
A new configuration for the RF power supply changes how the power supply
reacts to a rapidly changing nonlinear impedance. The new configuration
power supply makes the maximum forward power available from the unit close
to the perfect match point of 50 ohms real resistance. This stops the
power supply from delivering a higher power when the plasma impedance
changes, so the interaction oscillation never gets initiated.
In general, in one aspect, the invention is an RF power supply including a
splitter, two branch circuits, and a combiner. The splitter has an input
line receiving an RF signal, a first output line carrying a first output
signal derived from the RF signal, and a second output line carrying a
second output signal derived from the RF signal. A first branch circuit
receives the first output signal and generates therefrom a first derived
signal. A second branch circuit receives the second output signal and
generates therefrom a second derived signal therefrom. The first branch
circuit includes a first power amplifier and a phase shifting element. The
second branch circuit includes a second power amplifier. The combiner,
which has a first input receiving the first derived signal and a second
input receiving the second derived signal, combines the first and second
derived signals to produce a power output signal therefrom. The phase
shifting element is connected between the output line of the first power
amplifier and the first input of the combiner and produces a phase shift
in a signal passing from the first power amplifier to the combiner
relative to a signal passing from the second power amplifier to the
combiner.
In general, in another aspect, the invention is using the above described
power supply to supply power to a plasma processing chamber.
Preferred embodiments include the following features. The phase shift at
the output of the first amplifier is sufficient to prevent an oscillatory
instability in the RF power supply when supplying power to a plasma. More
specifically, the phase shift is about 90.degree.+n(180.degree.), where n
is an integer and assuming that a 0.degree. combiner is used). The first
branch circuit further includes a second phase shifting element connected
between the first output of the splitter and the first power amplifier.
The second phase shifting element produces a second phase shift which when
added to the phase shift of the first mentioned phase shifting element
causes the first derived signal to have a predefined phase relationship
with respect to the second derived signal. The predefined phase
relationship is determined by the requirements of the combiner. When using
a 0.degree. power combiner and splitter, the first mentioned phase shift
and the second phase shift cause the first derived signal to be in phase
with the second derived signal. For example, the first mentioned phase
shift is about 90.degree.+n(180.degree.)+m(360.degree.), where n and m are
integers, and the second phase shift is about
270.degree.+n(180.degree.)+k(360.degree.), where k is an integer. When
using a 180.degree. power combiner and a 0.degree. splitter, the first
mentioned phase shift and the second phase shift cause the first derived
signal to be 180.degree. out of phase with the second derived signal. For
example, the first mentioned phase shift is about
90.degree.+n(180.degree.)+m(360.degree.), where n and m are integers, and
the second phase shift is about 90.degree.+n(180.degree.)+k(360.degree.),
where k is an integer.
Also in preferred embodiments, the phase shifting elements are segments of
cable having predetermined lengths.
In general, in yet another aspect, the invention is a method of amplifying
an RF signal. The method includes the steps of: splitting the RF signal
into a first output signal and a second output signal; power amplifying
the first output signal to produce a first amplified signal; power
amplifying the second output signal to produce a second amplified signal;
phase shifting the second amplified signal relative to the first amplified
signal to produce a phase shifted signal; and combining the first
amplified signal and the phase shifted signal to produce a power output
signal therefrom.
The invention is simple to implement and it solves the oscillation problem
that often occurs particularly in RF power supplies that supply power to a
plasma in a plasma chamber.
Other advantages and features will become apparent from the following
description of the preferred embodiment and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block diagram of an RF plasma system;
FIG. 2 shows a typical single stage RF transistor power amplifier and load;
FIG. 3a shows the forward power output of a typical RF transistor amplifier
versus load impedance;
FIG. 3b is a Smith chart;
FIG. 4 shows a configuration of a typical high power RF power supply;
FIG. 5 shows an RF power supply constructed in accordance with the
invention;
FIG. 6 shows the forward power output contour for the power supply of FIG.
4;
FIG. 7 shows an embodiment of the invention which employs four power
amplifiers; and
FIG. 8 shows another embodiment using a 180.degree. power combiner.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, the basic elements of a typical RF plasma processing
system, such as might be used to fabricate semiconductor devices, include
an RF power supply 2, a plasma chamber 4, and an RF match circuit 6. The
RF power supply delivers an RF power signal over a coaxial cable 8 to
plasma chamber 4 to generate a plasma. The cable is connected to the
plasma generating elements (e.g. a coil or electrodes) within the plasma
chamber through the match circuit which is preferably mounted right on the
chamber close to the generating elements. The match circuit matches the
impedance of the plasma chamber to the output impedance of RF power supply
and the impedance of the coaxial cable, which is typically 50 ohms.
Maximum power transfer into the plasma within the chamber occurs when the
impedance seen by the output of the power supply is equal to 50 ohms real
(i.e., purely resistive). If the impedance seen by the power supply
through the coaxial cable is not the characteristic impedance of the
system, e.g. 50 ohms, a mismatch will exist and some of the power sent to
the chamber will reflect back to the power supply.
The match circuit, which is a conventional design, includes variable
reactive elements that can be adjusted to achieve the desired match
condition. A detector circuit 10, which is located within the unit that
contains the match circuit, monitors the voltages and currents in the
match circuit to determine whether the match circuit has achieved an
optimum match condition. Typically, it does this by sampling the input
current and input voltage of the match circuit and from these determine
input impedance. When the detector circuit senses that the match is not
optimum, it generates a signal which changes the reactive elements within
the match circuit to move the circuit toward the optimum match condition.
The oscillation phenomenon that occurs between the RF power source and the
plasma in the RF excited plasma chambers is not yet completely understood,
but we have shown it to be caused in part by the nonlinear plasma
impedance modulating the forward power of the RF power supply. As
indicated above, in RF excited plasma systems, the impedance of the plasma
can change rapidly and as the power supplied to the plasma changes that
too can cause the plasma impedance to change. These changes in the load
impedance of the power supply will, in turn, cause the RF power supply's
output power to change. If the slope of the plasma impedance nonlinearity
as a function of power is in the proper regime with respect to the RF
power supply, the change in plasma impedance can cause a corresponding
increase in the output power of the power supply which may cause a fast
"run-away" scenario. The only control over this condition is the forward
power control loop or mismatch protection circuit of a typical RF power
supply which will eventually respond according to it's loop time constant,
pulling back on the power supply output. The plasma impedance then returns
to the starting point, and the process begins again, sustaining a
power/impedance oscillation. The oscillation can also occur at frequencies
determined by the amount of stored energy in elements of the power supply,
which limit the amount of instantaneous energy available for the support
of the "run-away" condition.
Since the phase and the slope of the plasma nonlinearity as seen by the
power supply is a major factor, the existence of the oscillation is a
function of the coaxial cable length (i.e., electrical length) between the
plasma and the power supply. This has been shown repeatedly in experiments
where the RF system for the chamber is stable only at specific cable
lengths. Unfortunately, the cable length that is required for stable RF
operation is a function of the RF matching network, the characteristics of
the plasma impedance, and the characteristics of the power supply.
Therefore, the optimum length is not constant from system to system. This
poses a problem in a production environment.
Before describing the internal structure of the power supply design which
solves the oscillation problem described above, it is useful to first
obtain a more complete understanding of what causes the instability which
the invention eliminates. To this end, we refer to FIG. 2 which shows the
basic structure of an RF power supply 20. Power supply 20 includes a RF
signal source 22 which generates an RF signal and a power amplifier 24
which amplifies the RF signal to the levels required to produce a plasma
in the chamber. In this figure and in the remaining figures, the plasma in
the plasma chamber is modeled as a load, Z.sub.L. In its simplest form,
power amplifier 24 might be a single stage RF transistor power amplifier.
The forward power, P.sub.FWD, is defined as the power incident to the
load, Z.sub.L. For such a system, typical characteristics of the forward
power versus Z.sub.L are presented in the Smith chart shown in FIG. 3a.
As background, the Smith chart is a convenient tool widely used by persons
skilled in the art to analyze RF transmission line circuits. In short, it
is a graphical plot of normalized resistance and reactance functions in
the reflection coefficient plane (i.e., the .gamma.-plane). The reflection
coefficient, .gamma., is defined as the ratio of the complex amplitudes of
the reflected voltage and incident voltage at a load Z.sub.L that
terminates a loss-less transmission line having a characteristic impedance
of Z.sub.O. Mathematically, it is expressed as follows:
##EQU1##
where j=(-1).sup.1/2. The Smith chart, an example of which is shown in
FIG. 3b, is a plot in .gamma.-space, with the horizontal axis representing
.gamma..sub.r and the vertical axis representing .gamma..sub.j. Another
useful quantity, which is referred to as normalized load Z.sub.L /Z.sub.O,
is equal to:
##EQU2##
Different values of r when plotted in the Smith chart appear as a family
of circles of different radii with their centers located along the
.gamma..sub.r axis and all passing through .gamma.=1.0 .angle.0.degree..
Different values of x when plotted in the Smith chart appear as another
family of circles of different radii with their centers located along the
.gamma..sub.r =1 line and all passing through .gamma.=1.0. Thus, the Smith
chart makes it very easy to map load impedance into reflection coefficient
and vice versa. As will become apparent shortly, it also makes it very
easy to quickly determine the effect that a delay line or a phase shifter
has on the impedance seen by the power amplifier.
With that background, we return to power curve shown in FIG. 3a. The center
of the Smith chart is equal to the transmission line characteristic
impedance and power supply output impedance Z.sub.O , which in this case
is 50 ohms real. The vertical axis, which is perpendicular to the
.gamma.-plane (also referred to as the Z-plane), is the P.sub.FWD axis. A
plot of the forward power, P.sub.FWD, as a function of load impedance,
Z.sub.L, is a generally inclined, relatively planar surface 50, as shown
in the figure. At the match point where Z.sub.L =Z.sub.O, the forward
power delivered by the power amplifier is shown as the point labeled 52,
i.e., the P.sub.FWD axis intersects the power curve 50. As can be seen,
the match point is not the point of maximum forward power. In fact, the
forward power can increase significantly when Z.sub.L varies in certain
directions away from the 50 ohm real impedance value. It is this
characteristic which can lead to the unstable conditions that were
previously described.
Achieving the high power levels that are typically used in plasma
processing chambers often requires the combining of multiple stages of RF
amplifiers in parallel. An example of a conventional parallel
configuration involving two stages is illustrated in FIG. 4. As shown, two
identical power amplifiers 60a and 60b are linked together using a power
splitter 62 and a power combiner 64. (For a description of the design and
operation of power combiners and power splitters see standard textbooks,
e.g. Single-Sideband Systems and Circuit, Ed. William E. Sabin and Edgar
O. Schoenke, McGraw-Hill Book Company, pages 425-447).
Power splitter 62 receives an RF signal, P.sub.IN, from the RF source and
generates therefrom two identical signals of equal power. Each of power
amplifiers 60a and 60b amplifies a corresponding one of the signal coming
from power splitter 62. The outputs of the amplifiers are then combined in
power combiner 64 to produce an output signal with power equal to the sum
of the output powers of both power amplifiers 60a and 60b.
In the described embodiment, power combiner 64 is the same type of device
as power splitter 62, except operated in reverse. That is, the input lines
of the power combiner become the output lines of the power splitter and
the output line of the power combiner becomes the input line of the power
splitter. These devices also include a power resistor R.sub.1 connected to
a third port. The power combiner combines its two input signals equally
with any amplitude or phase imbalance being absorbed into power resistor
R.sub.1. In FIG. 4, both power splitter 62 and power combiner 64 are
0.degree. degree devices. That is, the signals appearing on the two lines
are in phase with each other. A 180.degree. device is also commercially
available. Regardless of which type is used, however, the impedance seen
by the output of power amplifier 60a is identical to the impedance seen by
the output of power amplifier 60b. Thus, when using a 0.degree. device or
a 180.degree. device, the forward power versus Z.sub.L characteristics for
both amplifiers 60a and 60b are the same as for the single stage shown in
FIG. 3. Thus, if a change in plasma impedance causes an increase in power
output of one amplifier, it will also cause a power increase in the other
amplifier.
Combining the RF amplifiers in this way has been found to cause the RF
power supply-plasma interaction oscillation mentioned above. Because the
forward power can increase in both branches when the plasma impedance
changes in certain directions, the oscillation can be initiated.
The potential for instability is completely eliminated by the modifications
shown in the circuit of FIG. 5. The modified circuit is identical to the
circuit of FIG. 4, except for the addition of phase shifting elements 72
and 74 before and after power amplifier 60a. More specifically, in the
described embodiment, a 270.degree.+n(180.degree.)+m(360.degree.) phase
shifting element 72 is added before power amplifier 60a and a
90.degree.+n(180.degree.)+k(360.degree.) phase shifting element 74 is
added after power amplifier 60a, where n, m, and k are integers which may
take on any positive value or zero. Phase shifting element 74 rotates the
impedance seen by power amplifier 60a relative to the impedance seen by
power amplifier 60b. Phase shifting element 72 adds sufficient phase shift
to the signal in the upper branch to produce a signal at the input of
power combiner 64 that is in phase with the input signal from the lower
branch (i.e., the phase difference between the two inputs is integer
multiples of 360.degree.).
Note that phase shifting element 74 could have been inserted before power
amplifier 60b instead. In that case, it would need to introduce a phase
shift of 90.degree.+n(180.degree.)+m(360.degree.) to satisfy the phase
requirements at the input of power combiner 64.
The phase shifting elements 72 and 74 can be simply implemented by using a
section of cable having the appropriate length (e.g. a cable having a
length of approx. 30" produces a 90.degree. phase shift for a 13.56 MHZ
signal).
In the configuration of FIG. 5, if Z.sub.L is exactly 50 ohms real, both
power amplifiers 60a and 60b will see a load impedance of 50 ohms real.
However, if Z.sub.L is not 50 ohms real (i.e., if the system has moved
away from a perfect match condition), power amplifier 60a will see an
impedance that is 1/4 wavelength (i.e., 90.degree.) out of phase with the
impedance seen by power amplifier 60b. More significantly, the power curve
of power amplifier 60a will also be 1/4 wavelength out of phase relative
to the power curve of power amplifier 60b. In other words, if we assume
that the power curve for power amplifier 60b is as shown in FIG. 3a, then
the power curve for power amplifier 60a will be rotated by 180.degree.
about the power axis (i.e., two times 90.degree.). Now, a change in
impedance away from the match condition will not cause both power
amplifiers to increase their power outputs. If a change in plasma
impedance causes the output power of power amplifier 60b to increase, it
will cause the output power of power amplifier 60a to decrease. This is a
characteristic of all ordinary class A, B, and C power amplifiers so long
as the output amplifier output is not limited by the current or voltage
capability of the amplifier's power supply. In other words, one power
amplifier will tend to compensate for the potentially destabilizing
response of the other power amplifier.
Power combiner 64 further assists in stabilizing the power supply in the
following way. When the two power amplifiers 60a and 60b produce signals
that have equal amplitude, are in phase, and thus have equal forward
power, power combiner 64 combines the two signals to produce an output
signal having a total forward power that is the sum of the forward power
of the two power amplifiers. Since the two input signals to power combiner
64 are in phase and of equal amplitude, power resistor, R.sub.1,
dissipates no power. However, when the two power amplifiers produce
signals that are not in balance, the amplitude imbalance will be absorbed
in power resistor, R.sub.1.
The net effect of phase shifting element 74 and power combiner 64 is to
produce a forward power versus Z.sub.L characteristic that appears as
shown in FIG. 6. The resulting power curve for the power supply exhibits a
maximum 80 at or near the optimum match point. It should be noted,
however, that depending upon the particular characteristics of the power
amplifiers that are used, a maximum may not actually be located at the
match point. Nevertheless, the resulting power curve will have a shape in
the vicinity of the optimum match point such that any movement away from
the optimum match condition will not cause the power output of the supply
to increase significantly (i.e., it will not increase enough to cause an
instability of the type previously observed). Indeed, this technique has
proven to completely solve the power supply-plasma interaction oscillation
problem described above.
The configuration of FIG. 5 is not the only configuration which
accomplishes this objective. The key is adding a sufficient phase shift at
the output of one power amplifier. As long as the impedances seen by the
outputs of two power amplifier stages are sufficiently out of phase, this
technique works to eliminate instabilities of the type previously
described. In other words, the phase shift at the output of the power
amplifier 60b need not be 90.degree.. It can be any amount which causes
that power amplifier to compensate for the potentially destabilizing
response of the other power amplifier. The amount can be determined
empirically for the specific power amplifiers that are used and for the
specific configuration. The second phase shift provided by phase shifting
element 72 simply satisfies a requirement that exists at the inputs of a
0.degree. combiner, namely, that they must be in phase with each other.
In addition, the technique may also be used in power supplies having more
than two stages. If more than two power amplifiers are used, they can be
grouped into two groups and each group can then be handled as a single
power amplifier of a two stage configuration such as was described above.
Alternatively, the power amplifiers can be treated individually. For
example, if there are four power amplifiers, such as is shown in FIG. 7,
phase shifting elements of 45.degree., 90.degree., and 135.degree. can be
used at the outputs of three of the power amplifiers. The phase shifting
elements operate to rotate the power curve by 90.degree. of one power
amplifier relative to its neighbor.
In general, if x stages are used (where x is typically an even integer),
then one approach is to use phase shift elements of r(180.degree./x),
where r identifies the stage and r=1,2, . . . , x.
In another embodiment shown in FIG. 8, a 180.degree. power combiner 65 is
used. In that case, a 90.degree.+n(180.degree.)+m(360.degree.) phase
shifting element is used in front of power amplifier 60a replacing the
270.degree.+n(180.degree.)+m(360.degree.) phase shifting element
previously described. This is necessary since the signals arriving at the
inputs of the 180.degree. power combiner must be 180.degree. out of phase.
It should also be possible to use two power amplifiers of different design,
each having a power curve that compensates for the power curve of the
other amplifier.
Typical power levels that are used in the described embodiment are 1 kW at
13.56 MHZ. The technique, however, works for any frequencies (e.g. RF) and
power levels that might be used in a plasma chamber.
Note that for the embodiments described above it should be understood that
any connection between the output of the power splitter and the input of a
power amplifier or between the output of the power amplifier and the input
of the combiner will typically introduce a phase shift in the signal. The
amount of phase shift that is introduced will either be small or large
depending upon the length of the circuit. Thus, implicit in the figures is
a phase shift elements at both the input and outputs of all amplifiers in
all of the branches. These are not shown, however, to simplify the
description and to highlight that it is the relative difference in phase
shift that is important.
Other embodiments of the invention are within the following claims.
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